# std::input_iterator

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< cpp‎ | iterator

Defined in header `<iterator>` |  |
template< class I > concept input_iterator = std::input_or_output_iterator<I> && std::indirectly_readable<I> && requires { typename /*ITER_CONCEPT*/<I>; } && std::derived_from</*ITER_CONCEPT*/<I>, std::input_iterator_tag>; |  | (since C++20)

The `input_iterator` concept is a refinement of `input_or_output_iterator`, adding the requirement that the referenced values can be read (via `indirectly_readable`) and the requirement that the iterator concept tag be present.

### Iterator concept determination

Definition of this concept is specified via an exposition-only alias template /*ITER_CONCEPT*/.

In order to determine /*ITER_CONCEPT*/<I>, let ITER_TRAITS<I> denote I if the specialization std::iterator_traits<I> is generated from the primary template, or std::iterator_traits<I> otherwise:

- If ITER_TRAITS<I>::iterator_concept is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
- Otherwise, if ITER_TRAITS<I>::iterator_category is valid and names a type, /*ITER_CONCEPT*/<I> denotes the type.
- Otherwise, if std::iterator_traits<I> is generated from the primary template, /*ITER_CONCEPT*/<I> denotes std::random_access_iterator_tag.
- Otherwise, /*ITER_CONCEPT*/<I> does not denote a type and results in a substitution failure.

### Notes

Unlike the LegacyInputIterator requirements, the `input_iterator` concept does not require `equality_comparable`, since input iterators are typically compared with sentinels.

### Example

A minimum input iterator.

    #include <cstddef>
    #include <iterator>

    struct SimpleInputIterator
    {
        using difference_type = std::ptrdiff_t;
        using value_type = int;

        int operator*() const;

        SimpleInputIterator& operator++();
        void operator++(int) { ++*this; }
    };

    static_assert(std::input_iterator<SimpleInputIterator>);

### See also

input_or_output_iterator (C++20) | specifies that objects of a type can be incremented and dereferenced (concept)
forward_iterator (C++20) | specifies that an `input_iterator` is a forward iterator, supporting equality comparison and multi-pass (concept)

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